This invention relates generally to a method and apparatus for compression forming both a support flange and a finish on a thermoplastic parison. More specifically, the method relates to sequentially compressing the opposed ends of the parison while supported within a forming mold to sequentially form the finish and then the support flange.
Over the past several years it has become possible and commercially desirable to form blown plastic bottles of reduced wall thickness due to increased material costs and due to the advent of molecularly orienting the plastic materials. However, in some cases bottles formed of this greatly reduced wall thickness do not have a stacking strength that is comparable either to the previous, thicker bottles or to glass bottles. This has posed various problems, such as in filling and capping operations when the axial strength of the bottle is critical to prevent collapsing.
In an attempt to overcome these specific problems, bottles have been made with an annular neck support ledge which is grasped by various conveying and supporting devices so that the bottle can be pendently supported during filling and capping to take the axial load off the bottle during these operations. In fact, this particular arrangement has solved the collapsing problem. However, because of various problems encountered, the molding art has not heretofore provided a method and apparatus for forming both the bottle finish and the support flange in a single operation. One problem is a resistance of certain plastic materials to flow into both the finish forming cavity and the flange forming cavity in a single operation, thus leaving an incompletely formed bottle. This problem is accentuated with certain plastic materials, such as polyethylene terephthalate which crystallizes at elevated temperatures and thus cannot be heated to improve the flowability of the plastic during the compression molding operations.